1. Field of the Invention
The present invention relates to air brake systems such as mounted on a vehicle and, more particularly, to an antilock system for use with such air brake systems.
2. Description of the Prior Art
One known form of air brake system (as disclosed in U.S. Pat. No. 4,399,894) is equipped with a disc brake-type brake system actuator having a power screw, with the braking force being applied by forward rotation of the power screw and released by backward rotation thereof. The air brake system is further equipped with a penumatic actuator for rotating the power screw forwardly and backwardly. More specifically, such pneumatic actuators are conventionally partitioned into two pressure chambers by means of a power piston and a diaphragm so that the power piston is reciprocated by varying the air pressure differential between the two chambers. An actuator rod joined to that power piston is connected to the brake arm of the power screw so that the power screw may be rotated forwardly or backwardly by the reciprocal movement of the actuator rod.
Another air brake system of the general type is disclosed in U.S. Pat. No. 3,997,035 to include a drum brake which generates a braking force by rotating a cam shaft.
Vehicles equipped with such air brake systems, however, may have one or more of their wheels locked while being braked, as where the road has a low coefficient of friction. In such case, it is desired to release the braking force so that the wheel or wheels may be unlocked, as is accomplished in the conventional hydraulic control-type brake systems. In order to prevent undesirably long braking distances, however, it is desirable to restore the braking force after the wheel or wheels are unlocked.
A braking system using such an "antilock" concept is disclosed in U.S. Pat. No. 3,929,383. The brake system thereof is controlled by varying the pressure difference between the two air chambers of a pneumatic actuator, as described above. However, to function properly, this braking system requires that the release of the braking force for effecting the antilock must be conducted very promptly, and because of the required pressure differential between the two air chambers, it has been found to be very difficult to ensure reliable, rapid response operation.